General and Synthetic Methods: Volume 8 (Specialist Periodical Reports - General and Synthetic Methods) - Hardcover

Hardcover

ISBN 10: 0851868940 ISBN 13: 9780851868943

Publisher: Royal Society of Chemistry, 1986

Synopsis

Reflecting the growing volume of published work in this field, researchers will find this book an invaluable source of information on current methods and applications.

About this title

From the Back Cover

A series of reviews by leading specialists in their fields which gives systematic and comprehensive coverage of the progress in major areas of research.

Excerpt. © Reprinted by permission. All rights reserved.

General and Synthetic Methods Volume 8

A Review of the Literature Published During 1983

By G. Pattenden

The Royal Society of Chemistry

Copyright © 1986 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85186-894-3

Contents

Chapter 1 Saturated and Unsaturated Hydrocarbons By J. M. Clough and C. R. A. Godfrey, 1, 
Chapter 2 Aldehydes and Ketones By S. C. Eyley, 84, 
Chapter 3 Carboxylic Acids and Derivatives By D. W. Knight, 131, 
Chapter 4 Alcohols, Halogeno-compounds, and Ethers By L. M. Harwood, 200, 
Chapter 5 Amines, Nitriles, and Other Nitrogen-containing Functional Groups By S. G. Lister, 245, 
Chapter 6 Organometallics in Synthesis, 312, 
Chapter 7 Saturated Carbocyclic Ring Synthesis By T. V. Lee, 381, 
Chapter 8 Saturated Heterocyclic Ring Synthesis By K. Cooper and P. J. Whittle, 407, 
Chapter 9 Highlights in Total Synthesis of Natural Products By K. E. B. Parkes and G. Pattenden, 497, 
Reviews on General and Synthetic Methods By G. Pattenden and G. M. Robertson, 541, 
Author Index, 548, 

CHAPTER 1

Saturated and Unsaturated Hydrocarbons

BY J. M. CLOUGH AND C. R. A. GODFREY

1 Saturated Hydrocarbons

Many new methods for the preparation of alkanes via reductive removal of functional groups have been reported during the year. A preparatively useful method for the conversion of carboxylic acids into alkanes has been developed by Barton and his co-workers. Primary, secondary, and tertiary carboxylic esters (1) derived from thiohydroxamic acids such as N-hydroxypyridine-2-thione undergo efficient radical chain decarboxylation to the corresponding nor-alkanes on treatment with either tri-n-butyltin hydride or t-butylmercaptan. Under these mild reaction conditions, ketones, olefins, and normal carboxylic esters remain unchanged. Decarboxylation of carboxylic acids can also be effected using sodium persulphate and a catalytic amount of silver nitrate. Unstabilized alkyl radicals from aliphatic acids afford alkanes or, in the presence of copper(II) salts, alkenes. By contrast, arylacetic acids give benzylic radicals, and these dimerize to give 1,2-diarylethanes in moderate yields.

Williams and Moore have reported that the reduction of a variety of heterocyclic thiones to the corresponding methylene compounds occurs readily on heating with an excess of tri-n-butyltin hydride and a radical initiator (e.g. Scheme 1). Conversion of the cyclic thiocarbonate (2) into the 1,3-dioxolane (3) is noteworthy in that the Corey–Winter reaction does not take place under the reaction onditions. The desulphurization of thiols and thioketones to alkanes and alkenes using sodium triethylborohydride and iron(II) chloride is improved by adsorption of the borohydride onto alumina. Moreover, this heterogeneous reaction occurs at room temperature and products are easily isolated by simple filtration. A variety of benzylic di- and tri-arylmethyl mercaptans react with stoicheiometric amounts of [Fe3(CO)12] or [Co2(CO)8] under phase-transfer conditions to give the corresponding hydrocarbons in good yields.

Monosubstituted thiiranes are reduced to alkanes on treatment with Raney nickel in ethanol at –40°C, conditions under which olefinic bonds are unaffected.

On irradiation, solutions of diselena[3.3]cyclophanes in HMPA are transformed cleanly into the corresponding cyclophanes (e.g. Scheme 2).

Treatment of (hydroxymethyl)diphenylphosphine oxides (4) with P2I4 in carbon disulphide at room temperature affords excellent yields of the alkyldiphenylphosphine oxides (5) with no trace of the corresponding iodides.

Suzuki and his co-workers have shown that benzyl alcohols are smoothly deoxygenated on treatment with P2I4 in boiling benzene. The reaction works well even with sterically hindered secondary benzyl alcohols, as illustrated in Scheme 3. Extending this work, the same group has reported that a mixture of LiAlH4 and P2I4 offers a mild alternative to conventional methods for the deoxy-genation of aromatic ketones (e.g. Scheme 4). Halogens, esters, and olefinic bonds are not affected by these reaction conditions.

Ueno and his co-workers have described conditions under which tosylates, including those derived from primary alcohols, undergo efficient radical deoxy-genation to give hydrocarbons. A noteworthy example is the selective removal of the tosyl group from the diol derivative (6) which takes place without the need to protect the free hydroxy-group.

Several reports describing the use of lithium triethylborohydride for the reduction of alkyl halides, especially alkyl fluorides, have been published during the year. Catalytic amounts of silver perchlorate markedly accelerate the reduction of 1,1-dibromocyclopropanes to the corresponding monobromides by LiAlH4. This catalyst also facilitates the reduction of tertiary or sterically hindered alkyl bromides which are normally resistant to LiAlH4. gem-Bromochloro-cyclopropanes react with a mixture of diethyl phosphonate and triethylamine to give the corresponding chlorocyclopropanes exclusively, and (trichloromethyl)-benzene is reduced to (dichloromethyl) benzene in a yield of 86% under the same conditions. Photostimulated reduction of either cyclohexyl chloride or bromide with LiAlH4 in the presence of di-t-butyl peroxide gives cyclohexane in good yield. Vinyl bromides are converted into olefins under these conditions, but yields are only moderate. α-Halogenocarbonyl compounds are smoothly dehalogenated on treatment with sodium hydrogen telluride, generated in situ from tellurium and NaBH4 in ethanol.

α-Nitrohydrazones (7), readily prepared from the corresponding nitroalcohols (8), are cleanly reduced to the hydrazones (9) on treatment with LiAlH4. However, the reaction fails for nitrohydrazones (7) in which R2 and R3 are both hydrogen atoms.

Dimeric products often encountered during the reduction of nitroalkenes to the corresponding nitroalkanes with NaBH4 can be avoided by carrying out the reaction at 25°C in the presence of silica gel, in a mixture of chloroform and propan-2-ol. Reasonable yields of alkanes can be obtained by electrohydrogenation of both alkenes and alkynes using a nickel-plated cathode coated with Raney nickel powder. However, many other functional groups are also reduced under these conditions.

Alper and Heveling have reported the first examples of organometallic phase-transfer catalysis under acidic conditions. For example, hydrogenation of 9,9'-bifluorenylidene (10) or diarylethylenes occurs on treatment with [Co2(CO)8] or [Co2(CO)6(PBu3)2] and tetrafluoroboric acid under phase-transfer conditions. Anthracene, however, is not reduced. The solvated ion pair [(C8H17)3NMe]+[RhCl4]- catalyses the hydrogenation of a variety ofonsaturated compouds under phase-transfer conditions. Even aromatic substrates may be reduced to the corresponding alkanes at room temperature and under a pressure of 2 atm of hydrogen, but reaction rates are sensitive to steric effects.

Reactive halides such as benzyl bromide undergo homo-coupling on treatment with titanocene methylene–zinc halide complexes of the type [Cp2TiCH2.ZnX2](X=Cl or I).

Reetz and Westermann have reported that treatment of lithium alkoxides of the type (11) with a 1:1 mixture of MeTiCl3 and Me2TiCl2 at –40°C affords the methylated products (12), which are potentially useful as synthetic tetra-hydrocannabinoid intermediates. Undesirable Wagner–Meerwein rearrangements or retro-Friedel–Crafts reactions are not observed under these reaction conditions. By contrast, gem-dimethylation of the optically active disubstituted cyclopentanone (13) using Me2TiCl2 affords a mixture of racemic cuparene (14) and the olefin (15), products which can be accounted for in terms of intermediate carbonium ions.

2 Olefinic Hydrocarbons

Although the extremely hindered olefin tetra-t-butylethylene has still not been synthesized, the closely related compounds (16), (17), (18), and (19) have been prepared and characterized, and some of these are potential precursors of tetra-t-butylethylene itself.

House and his co-workers have synthesized further examples of strained enones, either by elimination or by intramolecular Wadsworth–Emmons reactions. 2-Phenylbicyclo[3.3.1]non-1-en-3-one (20) is stable when protected from oxygen, or nucleophiles such as water. By contrast, the bicyclo[3.2.1]octane species (21) and (22) could only be generated as transient intermediates which were trapped with nucleophiles or, in the case of (22), as a cycloadduct with furan. A simple two-step preparation of bicyclo[3.3.0]oct-1-en-3-one also makes use of an intramolecular Wadsworth–Emmons reaction (potassium carbonate and 18-crown-6 in benzene at 60°C) to close the second ring.

Bicyclo[5.1.1]non-1(8)-ene (23) has been synthesized by Ramberg–Bäcklund reactions using each of the stereoisomeric bromosulphones (24) and (25) (Scheme 5). No competing 1,2-dehydrobromination is observed. Although isolable, the olefin (23) shows the usual high reactivity towards oxygen, acids, and dienes. Another way of constructing bridgehead olefins is via intramolecular Diels–Alder reactions, but high temperatures are usually required. Shea and Gilman have now shown that, in the presence of stoicheiometric amounts of diethylaluminium chloride, these reactions take place smoothly at room temperature. A particularly impressive example is the reaction shown in Scheme 6 which gives a 70% yield of the bridgehead olefin within 5 minutes.

Marshall and Flynn have reported that trans-hydroxymethylcycloalkenes [e.g. (26)] of known absolute configuration and high optical purity can be prepared by the Sharpless kinetic resolution procedure. The (+)-(R)-trans-cycloalkene (26) was converted in six steps into (+)-(R)-[10.10]-betweenanene (27) with better than 90% optical purity (Scheme 7).

The low-valent titanocene species generated by reducing [Cp2TiCl2] with sodium naphthalenide is a highly efficient and almost stereospecific catalyst for the isomerization of simple non-functionalized terminal alkenes to (E)-2-alkenes. The transformation is complete within minutes at room temperature and internal olefinic bonds of either configuration are inert under the reaction conditions. Hex-1-ene is rapidly converted into a stereoisomeric mixture of hex-2-enes on treatment with a catalytic amount of the stable and commercially available complex [OsHBr(CO)(PPh3)3] in toluene at 150°C.

Partial hydrogenation of acetylenes to olefins is often accomplished using the Lindlar catalyst. A systematic study with nine metal ions has now shown that the chemoselectivity for semihydrogenation, especially for monosubstituted acetylenes, is significantly and reproducibly improved when the catalyst is modified with manganese(II) chloride. Suzuki and his co-workers have described two new systems for the partial reduction of acetylenes to (Z)-olefins at room temperature. The first, a catalytic amount of palladium chloride in polyethylene glycol and dichloromethane, allows diphenylacetylene to be hydrogenated to (Z)-stilbene at atmospheric pressure, but over-reduction appears to be a problem. Much better selectivity is achieved with the second system in which NaBH4 replaces hydrogen. Simple 1,3-dienes can be hydrogenated specifically to olefins at –78°C using allyl(hydrido)platinum(II) complexes as catalysts.

Oxiranes are conveniently deoxygenated to give olefins using hydrogen iodide generated in situ from toluene-4-sulphonic acid and sodium iodide in acetonitrile. The transformation is complete within minutes at room temperature.

Vinyl sulphones are reduced to the corresponding olefins in high yield and with retention of configuration on treatment with alkyl Grignard reagents and nickel or palladium catalysts in THF at room temperature (e.g. Scheme 8). Coupling products between the sulphone and Grignard reagent are only formed to a small extent (generally <6%) under these conditions. Another new method for the reduction of vinyl sulphones to olefins is illustrated by the examples shown in Scheme 9. Its success stems from the regiospecific 1,4-addition of tributylstannyl-lithium to vinyl sulphones in THF at –78°C, and the instability of the resulting β-tributylstannylsulphones, which collapse to form olefins on treatment with silica gel. A drawback of the method is its lack of stereochemical control, but it does provide a chance to modify the product by treating the intermediate sulphonyl-stabilized anion with electrophiles such as methyl iodide (Scheme 9) or aldehydes (which give allylic alcohols).

Vinylsilanes with a hydroxy-group in the β-position are rapidly desilylated via a homo-Brook rearrangement on treatment with potassium hydride (e.g. Scheme 10).

A sequence for the reduction of a cyclopentenone to the corresponding cyclopentene without scrambling of the position of the olefinic bond has been devised as part of a synthesis of a prostaglandin analogue (Scheme 11).

Tritylpotassium rapidly dehydrohalogenates secondary alkyl bromides and iodides at 0°C to give high yields of olefins. The reagent is less satisfactory with secondary alkyl chlorides and it fails altogether with primary alkyl halides. When solutions of phenethyl or secondary alkyl bromides are treated with 2.5 equivalents of alumina-supported potassium fluoride, smooth dehydrobromination takes place, but without regio- or stereo-control. Under the same conditions, 1,2-dibromides and vinyl bromides are converted into acetylenes, and 2phenoxyethyl bromide gives phenyl vinyl ether.

In contrast to the well-known selenoxide elimination, little has been reported concerning olefin formation by elimination of telluroxides. Uemura and Fukuzawa have now shown that although primary alkyl phenyl telluroxides undergo elimination only at high temperatures, secondary alkyl phenyl telluroxides readily decompose at room temperature to give isomeric mixtures of olefins (e.g. Scheme 12). The use of appropriate precursors allows the reaction to be applied to syntheses of allylic alcohols and allylic and vinyl ethers, and this is of more preparative value since elimination is both regio- and stereo-specific in these cases (e.g. Scheme 13).

On heating in DMSO, 2-cycloalkoxytropones, prepared from cycloalkanols, tropolone, and DCC, undergo stereospecific trans-elimination to give high yields of cycloalkenes. Experiments with deuterium-labelled precursors support the concerted intramolecular mechanism shown in Scheme 14. The analogous tropone derivatives of acyclic alcohols give acyclic olefins under the same conditions, but these are contaminated with ketones derived from the original alcohols.

Reductive debromination of vic-dibromides to give olefins is conveniently conducted using sodium sulphide or sodium hydrogen sulphide under aqueous phase-transfer conditions. Elimination takes place with at least 85% anti-stereoselectivity. A variety of functional groups (hydroxyl, ketone, lactone) tolerate the reaction conditions, but olefinic bonds can migrate into conjugation with carbonyl groups. The same transformation takes place when benzene solutions of vic-dibromides are irradiated with u.v. light in the presence of three equivalents of triethylamine, but generally without stereocontrol. α,β-Dihalogeno-ketones and -esters are converted into α,β-unsaturated ketones or esters under the same conditions. Sodium hydrogen telluride reduces 1-nitro-1-(1-nitrocyclohexyl)cyclohexane, a vic-dinitro-compound, to cyclohex-ylidenecyclohexane in a yield of 86%.

Hindered methyl esters which possess a suitably placed benzylic or allylic bromine atom undergo fragmentation on heating in HMPA to give high yields of olefins (e.g. Scheme 15).

New experimental conditions for the Wittig reaction involve passing an aldehyde in a gaseous state through a heated bed of a phosphonium salt and potassium carbonate. The olefinic product, also a gas under the reaction conditions, is collected, leaving the triphenylphosphine oxide adsorbed on the solid bed. A wide range of aldehydes are suitable, provided they have sufficient volatility, but ketones do not react. Wittig reactions performed with reactive ylides under solid–liquid phase-transfer conditions (potassium carbonate in wet 1,4-dioxane or methanol) allow selective mono-olefination of terephthalic aldehyde or olefination of phenolic benzaldehydes without protection of the phenolic group. Furthermore, Wittig reactions between aldehydes and resonance-stabilized phosphoranes are strongly accelerated if performed under high pressure (10 kbar) and there is also an improvement in the (E)-stereoselectivity.

(Continues...)

Excerpted from General and Synthetic Methods Volume 8 by G. Pattenden. Copyright © 1986 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 1986
• Language: English
• ISBN 10: 0851868940
• ISBN 13: 9780851868943
• Binding: Hardcover
• Number of pages: 584
• Editor: G Pattenden